WASHINGTON -- When dozens of world leaders and top-level delegations from around the globe descend on the U.S. capital this week to discuss how to better protect nuclear materials from terrorists, officials from one nuclear power won’t be there.

Russia, which has the world’s largest nuclear arsenal and a sprawling military and civilian nuclear industry long plagued by security concerns, will not be represented at the Nuclear Security Summit that opens on March 31.

Its boycott of the summit, which will be the fourth and possibly last such high-level effort aimed at preventing a terrorist group from obtaining nuclear materials, has triggered confusion and concern among officials and analysts in the United States and elsewhere.

“Frankly, we’re scratching our heads a bit,” Rose Gottemoeller, the U.S. State Department’s top official for arms control treaties, said at a March 29 briefing in Washington.

Ben Rhodes, a top official on the White House National Security Council, said Russia's decision was a "missed opportunity."

"Frankly, all they're doing is isolating themselves in not participating, as they have in the past," Rhodes told reporters in a conference call.

Russia’s decision not to attend is “definitely a concern,” said Kingston Reif, a nuclear and defense analyst at the Arms Control Association, a Washington-based research organization.

“It’s not like Russia has decided that nuclear material security isn’t something it should be focused on, or something that it no longer takes seriously,” Reif said. “But at the same time it does raise questions about Russia, its willingness moving forward, to seek to work with other countries, including the United States…to ensure that terrorists can’t get their hands on radiological materials.”

Many nuclear security experts argue the summits have helped focus attention on the problem, and promoted concrete measures in dozens of countries.

That includes removing highly enriched uranium -- which can be used in manufacturing a fissile bomb similar to the Hiroshima weapon -- from 12 countries and taking them to places like Russia to be blended into less dangerous forms.

Summit organizers also point to two dozen research reactors around the world that were modified to use low-enriched uranium, and security upgrades at another 32 buildings in various countries where fissile material is stored.

Still, many other arms control experts, including those at the Project on Managing the Atom, at Harvard University’s Kennedy School, gave a tempered assessment of past summits and their achievements.

“Security for nuclear materials has improved modestly -- but the capabilities of some terrorist groups, particularly the Islamic State, have grown dramatically, suggesting that in the net, the risk of nuclear terrorism may be higher than it was two years ago,” Kennedy School researchers wrote in a report released this month.

It’s a danger that was highlighted last month after Belgian officials confirmed that a man with links to the Paris terrorist attackers in November had surveillance video of a top Belgian nuclear official. Just last week, Belgian officials stripped security badges from some workers at the country’s nuclear plants as a precaution.

Declining Ties

Among the reasons Harvard researchers cited in their less-than-enthusiastic conclusions was the decline of nuclear cooperation between Moscow and Washington.

Despite sending delegations to the three previous summits, Moscow announced last year that no Russian officials would be coming to Washington.

Foreign Ministry spokeswoman Maria Zakharova said in January that the summits interfered with international organizations like the International Atomic Energy Agency, the UN’s nuclear watchdog, and imposed on them the "opinions of a limited group of states."

That decision followed Russia’s announcement in early 2015 formally ending its participation in a two-decade-old, U.S.-funded program to scrap unneeded nuclear weapon systems and secure facilities where radiological material was stored.

Despite its billion-dollar price tag, the Cooperative Threat Reduction Program has been largely viewed as a success in patching up decrepit Soviet-era security and safeguarding Russian nuclear materials.

But the program included the presence of U.S. elected officials visiting secretive or formerly closed research and military facilities, something that many in the Russian defense establishment viewed with deep suspicion.

President Barack Obama, then a U.S. senator, participated in one such trip in 2005, when he and another senator were briefly detained by Russian security officials.

“The Russians made the determination that that effort…no longer was a model they wanted to continue, that they saw themselves now as a country that is able to take on this work on its own. And I think they also resented the fact that there were U.S. representatives inside Russia working on this, and they were concerned about their presence,” Reif told RFE/RL.

‘Mixed Picture’

In a commentary published March 28, Kelsey Davenport, a nonproliferation expert also with the Arms Control Association, said Russia’s absence from the summit was concerning, particularly in light of a recent incident in the former Soviet state of Moldova involving the smuggling of nuclear materials that may have originated in Russia.

However, she said it shouldn’t be interpreted as a drastic shift in thinking by the Kremlin.

Gottemoeller said that Washington and Moscow continue with some nuclear cooperation despite what she called the “severe crisis” over the conflict in Ukraine and Moscow’s annexation of Crimea.

Russia agreed to take nearly all of Iran’s low-enriched uranium to blend it down to less dangerous levels and was continuing to implement the New START treaty that made further cuts to the Russian and U.S. nuclear stockpiles, she noted.

“The best thing I can say is it’s a mixed picture. There are areas of very sound cooperation with Russia, where we are proceeding and continuing to make progress,” Gottemoeller said.